Comment by skew-aberration

1. The coral slowly starves to death. Like a tree without leaves 2. It's complicated but the algae essentially start damaging the coral in these conditions. Their photosynthesis mechanism runs into overdrive, generating too many electrons. With no way to consume this much power it dumps the energy into making reactive oxygen species which it ejects. These damage the coral (and the algae). The same thing occurs in plants - a fern growing on the forest floor can't handle full sunlight (photodynamic stress). Plants are less temperature sensitive though because they have active cooling (transpiration). But temperature and quantity of light both affect the photosynthesis machinery.